Doppler effect

Photo by: Uladzimir Bakunovich

The Doppler effect is an effect observed in light and sound waves as they
move toward or away from an observer. One simple example of the Doppler
effect is the sound of an automobile horn. Picture a person standing on a
street corner. A car approaches, blowing its horn. As the car continues
moving toward the person, the pitch of the horn appears to increase; its
sound goes higher and higher. As the car passes the observer, however, the
effect is reversed. The pitch of the car horn becomes lower and lower.

Explanation

All waves can be defined by two related properties: their wavelength and
frequency. Wavelength is the distance between two adjacent (next to
each other) and identical parts of the wave, such as between two wave
crests (peaks). Frequency is the number of wave crests that pass a given
point per second. For reference, the wavelength of visible light is about
400 to 700 nanometers (billionths of a meter), and its frequency is about
4.3 to 7.5 × 10
14
hertz (cycles per second). The wavelength of sound waves is about 0.017
to 17 meters, and their frequency is about 20 to 20,000 hertz.

The car horn effect described above was first explained around 1842 by
Austrian physicist Johann Christian Doppler (1803–1853). To
describe his theory, Doppler used a diagram like the one shown in the
accompanying figure of the Doppler effect. As a train approaches a
railroad station, it sounds its whistle. The sound waves coming from the
train travel outward in all directions. A person riding in the train would
hear nothing unusual, just the steady pitch of the whistle's sound.
But a person at the train station would hear something very different. As
the train moves forward, the sound waves from its whistle move with it.
The train is chasing or crowding the sound waves in front of it. An
observer at the train station hears more waves per second than someone on
the train. More waves per second means a higher frequency and, thus, a
higher pitch.

An observer behind the train has just the opposite experience. Sound waves
following the train spread out more easily. The second observer detects
fewer waves per second, a lower frequency, and, therefore, a lower-pitched
sound.

Words to Know

Hubble's law:
The law that shows how the redshift of a galaxy can be used to
determine its distance from Earth.

Redshift:
The lengthening of the frequency of light waves as they travel away
from an observer; most commonly used to describe movement of stars away
from Earth.

It follows from this explanation that the sound heard by an observer
depends on the speed with which the train is traveling. The faster the
train is moving in the above example, the more its sound waves are bunched
together or spread out—thus, the higher or the lower the pitch
observed.

Doppler effect in light waves

Doppler predicted that the effect in sound waves would also occur with
light waves. That argument makes sense since sound and light are both
transmitted by waves. But Doppler had no way to test his prediction
experimentally. Doppler effects in light were not actually observed, in
fact, until the late 1860s.

In sound, the Doppler effect is observed as a difference in the pitch of a
sound. In light, differences in frequency appear as differences in color.
For example, red light has a frequency of about 5 × 10
14
hertz; green light, a frequency of about 6 × 10
14
hertz; and blue light, a frequency of about 7 × 10
14
hertz.

Suppose that a scientist looks at a lamp that produces a very pure green
light. Then imagine that the lamp begins to move rapidly away from the
observer. The Doppler effect states that the frequency of the light will
decrease. Instead of appearing to be a pure green color, it will tend more
toward the red end of the spectrum. The faster the lamp moves away from
the observer, the more it will appear to be first yellow, then orange,
then red. At very high speeds, the light coming from the lamp will no
longer look green at all, but will have become red.

Applications

The green lamp example described above has been used to great advantage by
astronomers when observing stars. The light of a star as seen

The Doppler effect.
(Reproduced by permission of

The Gale Group

.)

from Earth is always slightly different from its true color because all
stars are in motion. When astronomers observe stars in our own Milky Way
galaxy, for example, they find that the color of some stars is shifted
toward the blue, while the color shift in other stars is toward the red.
Blueshift stars are moving toward Earth, and redshift stars are moving
away from Earth.

In 1923, American astronomer Edwin Hubble (1889–1953) made an
interesting discovery. He found that all stars outside our own galaxy
exhibit redshifts of light. That is, all stars outside our galaxy must be
moving away from Earth. Furthermore, the farther away the stars are, the
more their redshift and, thus, the faster they are moving away from us.

Hubble's discovery is one of the most important in all of modern
astronomy. It tells us that the universe as a whole is expanding. Like
dots on the surface of a balloon that's being blown up, galaxies
throughout the universe are racing away from each other. One conclusion to
be drawn from this discovery is that—at some time in the
past—all galaxies must have been closer together at the center of
the universe. Ever since that time, those galaxies have been moving away
from each other. This conclusion is the basis for the currently popular
theory about the creation of the universe, the big bang theory.

The Doppler effect has many other practical applications. Weather
observers can bounce radar waves off storm clouds. By studying the
frequency of the waves that return, they can determine the direction and
speed with which the clouds are moving. Similarly, traffic police use
radar guns to determine the speed of vehicles. The faster a car or truck
is traveling, the greater the change in the frequency of the radar waves
it reflects.

Sound waves are used for underwater observations. A submarine sends out
sound waves that are reflected off other underwater objects, such as
another submarine or a school of fish. The frequency of the reflected
sound tells the direction and speed of the other object.

I am writing a book for a major publisher and am referring to the Doppler effect. I have searched for an existing artwork of the famed Doppler train illustration. I have tried to locate the Gale Group to seek their permission to use this artwork (hopefully at print resolution). Can you help me find this group?

I have a hopeless year 10 Astronomy teacher who was unable to teach my about the Doppler Effect and this site has made it all clear! I finally understand the Doppler Effect fully and its relationship to astronomy
Thank you!

I was watching a t.v program and a person from the show was dressed as the dobler effect, i did'nt know what it was so l looked it up and found this information. It is very interesting as you are given other info also. Thank you

Is there any laboratory experiment which certifies that redshift is a Doppler efect? Because the proponent of the Electric Universe theory, especially the astronomer Alton Harp, have shown the opposite

I found this website verry useful. First time I'd visited this website was 2 years ago. And still I am visiting this site. I gain a lot of things about science and technology from this site. I would like to give thanks for this site.

Comment about this article, ask questions, or add new information about this topic: